1. Field of the Invention
The present invention relates to a printing apparatus and, particularly, to an inkjet printing apparatus configured to efficiently use a limited power supply capacity of the apparatus.
2. Description of the Related Art
An inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) prints by discharging ink from a printhead using an electrothermal transducer (heater) as a printing element. The printing apparatus prints an image on printing paper by heating the printing element in accordance with image data and discharging ink. To meet a requirement for shortening the printing time, recent printing apparatuses tend to employ large-scale printheads in which a larger number of printing elements are integrated at high density. Thus, the consumption power of the printhead tends to increase. Along with an increasing speed of a carriage which supports the printhead, it is becoming necessary to install a power supply having a large capacity in the printing apparatus.
The load of the printhead greatly changes depending on a printing image and printing mode. For example, even in the same printing mode, the count and frequency at which the printing elements of the printhead are driven differ between a low-density image and a high-density image, so the load of the printhead also changes. Even for the same printing image, the load of the printhead changes between a high carriage speed and a low carriage speed. When performing multi-pass printing, the load of the printhead greatly changes depending on the number of passes. If the power supply is designed in accordance with a possible maximum power load in consideration of these changes, it becomes very large and expensive.
In some cases, to prevent increases in the size and cost of the power supply, the printing apparatus executes printing operation control to suppress the power load to a predetermined level, and the power supply is designed in accordance with the power load.
To restrict the power load by controlling the printing operation, there is a method of detecting the driving count of printing elements in the printhead in association with the size of a partial region in the scan region, and when the driving count of printing elements is higher than a predetermined value, decreasing the carriage speed or performing divisional printing. This method controls driving of printing elements (see, for example, Japanese Patent Laid-Open No. 4-115950, Japanese Patent Publication No. 6-047290, Japanese Patent Laid-Open Nos. 2005-224955 and 2006-007759, and Japanese Patent Nos. 3179674 and 3376118). This control is also called power monitoring control. A printing element driving count proportional to a power load is detected before printing. When the count exceeds the suppliable power of the power supply, driving of printing elements is controlled to reduce the load. Only when the load of the printhead may exceed the suppliable power of the power supply, this control can reduce the load without excessively decreasing the printing speed. Hence, this method is effective for cost reduction of the power supply.
In general, the suppliable power of a power supply is also related to the temperature ratings of components installed in the power supply. The power supply is designed so that the temperatures of the components installed in the power supply do not exceed their temperature ratings when a rated power is kept supplied. Within the normally operable temperature range of the printing apparatus, the apparatus is expected to operate normally. Therefore, the power supply needs to be designed on the premise of a highest temperature within the normally operable temperature range. The component temperature is proportional to the power load of the power supply. When the ambient temperature of the power supply is low, a power load required for the component temperature to reach the temperature rating becomes higher than that when the ambient temperature is high.
However, the related arts adopt a fixed threshold regardless of the ambient temperature in order to limit the load on the printhead. Even when the ambient temperature is low, no large power can be output. If a larger power can be supplied, the power load of the printhead per unit time can be increased. As a result, the driving control count by power monitoring control is decreased, and an increase in printing speed is expected.